The best tool you can get is a factory service manual. However, by 1989 the systems on vehicles were so the factory service manual is a milti-volume set that can get expensive to buy, even used.
I was able to get the factory manual CD so I have everything in PDF form. Maybe I'm just really bad at digging through 4500 pages of information, but I was hoping to find a diagnosis guide for idle conditions like I found for everything else. Hopefully it just a matter of finding it.
Even with the manuals, that seems like a really good site - thanks.
(1) check the throttle position sensor. If you can, try to check it without removing the throttle body. Make sure it has a smooth change in voltage as you work the throttle.
It sounds like the process for this is to:
1. Make sure reference is good (5V)
2. Make sure ground is good
3. While adjusting the throttle, make sure the signal wire goes smoothly between ~1V and 5V?
With the specifics being to back-probe the respective wires with the TPS still hooked up.
The videos I've watched of people doing this have all had a throttle cable - it's not clear to me from the service manual if my engine is drive-by-wire or has a mechanical connection between the pedal and throttle body. I'm sure I can figure this out by looking once it stops raining.
(2) your van has a MAP sensor, not a MAF sensor. The batch fired injection used in Ford trucks and vans of that era was based on MAP, not MAF. The Mustangs got the sequential maf-based fuel injection. But do replace the MAP.
As soon as I locate it - I believe it's near the AC/heater housing inside the engine compartment?
(3) do check for vacuum leaks. You should get a vacuum gauge and have about 18-20 inches of vacuum at idle.
Will be doing this.
No fuel passes through the throttle body, so fuel additive will not clean it. Fuel additive might clean the injectors, but I doubt it. Fuel is a solvent. Fuel passes at high presure through the injectors the entire time the engine is running. There isn't much of a chance for buildup to form on the nozzles. The electromechanical parts can wear out, so maybe a new set of injectors is in order.
Could be - I was hoping to avoid this at least for a while.
Also check the condition of the timing chain (is the timing steady or does it fluctuate?) and make sure your base timing is set properly to 10 degrees BTDC (remove the SPOUT when you set it. Don't know what the SPOUT is? Check out that website I linked to).
I had the timing chain checked (but not the timing) when I had the water pump replaced a month ago. It was in good shape. I gave them the go-ahead to change it if needed and they said it was not needed. They did replace the cover with the pump, though.
The idle should be silky smooth and not surge like you are experiencing. I went crazy fixing lots of little things before I got my old 302 to idle right. However, I ended up replacing the 302 because it was just plain worn out. A van is a big heavy vehicle and the previous owner of my van used it to haul a 30 foot travel trailer around the country.
Thankfully this van doesn't have a hitch and has never towed anything. But that doesn't mean the engine was well cared for. I have no reliable information from the previous owner.
Thank you very much for the info - I'll start digging into this.